1. Field of the Invention
The present invention relates to a circuit board and a method for manufacturing the circuit board. More particularly, the present invention relates to a circuit board for transmitting electric signals at high speed and a method for manufacturing the circuit board.
2. Description of the Related Art
Accompanying the speeding up of networking equipments, a transmission in the high frequency region higher than 1 GHz is required in a large circuit board used in a high-end server or a communication base station. For the transmission in the high frequency region, a strip line is generally used in the circuit board.
FIG. 15 is a schematic cross-sectional view of an essential part for describing a wiring structure of a strip line.
A board having this wiring structure is a most basic unit for forming a multilayer circuit board. When forming the multilayer circuit board, these units are stacked into a multilayer.
The specific structure is as follows. That is, a wiring layer 100 as a central conductor and an insulating section 101 surrounding the wiring layer 100 are provided. Further, a power layer 102 and ground layer 103 having a width remarkably wider than that of the wiring layer 100 are provided on upper and lower surfaces of the insulating section 101, respectively.
Examples of the material for the wiring layer 100, the power layer 102 and the ground layer 103 include a copper foil. Examples of the material for the insulating section 101 include a dielectric material as an organic material. According to such a wiring structure, signals in a high frequency region can be transmitted by an electromagnetic field occurring between the wiring layer 100 and the power layer 102 or between the wiring layer 100 and the ground layer 103, namely, by a quasi-transverse electromagnetic wave.
However, in a signal transmission in a high frequency region, a transmission loss due to a resistance loss or a dielectric loss generally occurs. Particularly, it is known that the transmission loss depends on a frequency and increases in a high frequency region where a transmission speed is higher than 1 GHz. Therefore, a study of reducing a transmission loss in a high frequency region where a transmission speed is higher than 1 GHz is made in a circuit board.
For example, in a study of reduction in a resistance loss, there is studied reduction in a transmission loss by reduction in a surface roughness (see, e.g., Japanese Unexamined Patent Publication No. 2004-327745). However, this study has the following problems. For the reduction in a surface roughness, a complicated surface treatment technique is used, and therefore the cost increases for practical purposes. In addition, when the surface roughness is reduced, an adhesion in an interface between a wiring layer and an insulating section decreases. Accordingly, a predetermined surface roughness is required to maintain the adhesion in an interface between the wiring layer and the insulating section. Therefore, there is a limit in realizing, reduction in a resistance loss by reducing the surface roughness.
On the other hand, in a study of reduction in a dielectric loss, there is studied reduction in a transmission loss by reduction in a dielectric constant and dielectric tangent of an insulating section (see, e.g., Japanese Unexamined Patent Publication No. 2004-140268).
The dielectric constant and dielectric tangent of the insulating material are as follows. For example, a dielectric constant and dielectric tangent of Flame Retardant Type 4 (FR-4) generally used as a material for an insulating section of a circuit board are about 4.4 and 0.02, respectively.
To the contrary, a dielectric constant and dielectric tangent of the material mainly comprising polyphenylene ether (PPE) which is expected to be a material with a low dielectric constant and a low dielectric loss are about 3.5 and 0.002, respectively. Thus, the study of realizing a low dielectric constant and low dielectric loss of the insulating section is made to reduce a transmission loss in a high frequency region where a transmission speed is higher than 1 GHz.
However, when an insulating section material having a low dielectric constant and a low dielectric loss is used to realize the reduction of a transmission loss, other characteristics such as heat characteristics, interlayer adhesion, mechanical characteristics and chemical resistance of the insulating section material decreases due to decrease in a polarity of an organic material for forming the insulating section. Accordingly, when the insulating section material having a remarkably low dielectric constant and dielectric tangent is used for the circuit board, there arises a problem that deformation of the insulating section and peeling thereof from the wiring occur, for example, in a heating process of soldering. Particularly, the organic material has a tendency that when the dielectric tangent is reduced, the dielectric constant decreases at the same time. Therefore, there is a limit in the manufacture of a material, in which only dielectric tangent is reduced while allowing heat characteristics, interlayer adhesion, mechanical characteristics and chemical resistance to be maintained. For matching a characteristic impedance of wirings, it is not preferable to largely change the dielectric constant in the insulating section of the circuit board heretofore used. Further, the transmission loss must be sufficiently reduced.